Natural killer (NK) cells have been demonstrated to play a role in the rejection of bone marrow allografts in mice. We have found that NK cell subsets exist in their ability to reject bone marrow cells. The subsets can be distinguished on the Ly-49 receptor they express. Activated NK cells can also mediate antitumor effects in vitro and in vivo. We found that activated NK cells can also suppress graft-versus-host disease after allogeneic bone marrow transplantation in mice and still proved significant anti-tumor effects. Neuroendocrine hormones have been demonstrated exert immune effects in vivo and we have found that both growth hormone and prolactin can accelerate immune and hematopoietic recovery after syngeneic bone marrow transplantation in mice. NK cell function was markedly affected by prolactin treatment. CD40 is a molecule present on a variety of cell-types and has been demonstrated to play a critical role in B cell differentiation and function. CD40 has also been found to be present on various carcinomas but not normal tissue. We have found that CD40 stimulation by a recombinant soluble ligand can inhibit the growth of human breast cancer cell lines in vitro and in vivo. CD40 stimulation by the ligand can also accelerate immune and hematopoietic recovery after syngeneic bone marrow transplantation in mice.